Fix lifetime generics in <T<..> as Trait>::try_from suggestion.

Author: m-ou-se

compiler/rustc_typeck/src/check/method/prelude2021.rs

@@ -262,31 +262,35 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 method_name.name
             ));
 
-            let self_ty_name = self
+            let mut self_ty_name = self
                 .sess()
                 .source_map()
                 .span_to_snippet(self_ty_span)
                 .unwrap_or_else(|_| self_ty.to_string());
 
-            let self_ty_generics_count = match self_ty.kind() {
-                // Get the number of generics the self type has (if an Adt) unless we can determine that
-                // the user has written the self type with generics already which we (naively) do by looking
-                // for a "<" in `self_ty_name`.
-                Adt(def, _) if !self_ty_name.contains('<') => self.tcx.generics_of(def.did).count(),
-                _ => 0,
-            };
-            let self_ty_generics = if self_ty_generics_count > 0 {
-                format!("<{}>", vec!["_"; self_ty_generics_count].join(", "))
-            } else {
-                String::new()
-            };
+            // Get the number of generics the self type has (if an Adt) unless we can determine that
+            // the user has written the self type with generics already which we (naively) do by looking
+            // for a "<" in `self_ty_name`.
+            if !self_ty_name.contains('<') {
+                if let Adt(def, _) = self_ty.kind() {
+                    let generics = self.tcx.generics_of(def.did);
+                    if !generics.params.is_empty() {
+                        let counts = generics.own_counts();
+                        self_ty_name += &format!(
+                            "<{}>",
+                            std::iter::repeat("'_")
+                                .take(counts.lifetimes)
+                                .chain(std::iter::repeat("_").take(counts.types + counts.consts))
+                                .collect::<Vec<_>>()
+                                .join(", ")
+                        );
+                    }
+                }
+            }
             lint.span_suggestion(
                 span,
                 "disambiguate the associated function",
-                format!(
-                    "<{}{} as {}>::{}",
-                    self_ty_name, self_ty_generics, trait_name, method_name.name,
-                ),
+                format!("<{} as {}>::{}", self_ty_name, trait_name, method_name.name,),
                 Applicability::MachineApplicable,
             );